I could only dream of 20mpg in the 300SE but I never took it on a really long haul. In comparison, the S420 did 20-21mpg on an SJC-LAX roundtrip in the summer with AC blasting (outside temp gauge showed up to 107F) and cruise set at 85-90mph.

Sixto
95 S420
87 300SDL
83 300SD ... $1100, see cars forum